State-of-the-art pacemakers, ICDs and other cardiac rhythm management devices (CRMDs) can be equipped with radio-frequency (RF) communication devices for communicating with external systems such as bedside monitors or external diagnostics systems. In particular, RF communication devices have been developed to utilize Medical Implant Communication Service (MICS)-band radio transmissions or Medical Device Radiocommunications Service (MedRadio)-band transmissions. (MedRadio maintains the spectrum previously allocated for MICS (402-405 MHz) while adding additional adjacent spectrum (401-402 MHz and 405-406 MHz).) Herein, the term “MICS/MedRadio” will be used for the sake of completeness and generality to refer to MICS, MedRadio or both.) RF capable devices use an antenna within the header or adjacent header for receiving or transmitting RF signals. However, problems arise in designing such antennas due to the increasing miniaturization of CRMDs and their components.
In particular, there can be a loss of RF communication performance due to the reduction in size of the header and the device case (also called the housing or the “can”) of the CRMD. As technology improves, the sizes of the implantable devices continue to shrink but the laws of physics regarding RF communications do not change. Since about 2005, at least some CRMD designers have employed a shorted loop antenna for RF communications. However, RF computer simulations indicate that a further reduction in device size would diminish antenna performance below acceptable levels. Accordingly, there is a need to provide improved antenna designs for use with CRMDs, especially relatively small devices.
In this regard, there are many challenges to designing a well performing antenna for use within an implantable medical device. One issue is the significant amount of attenuation inherent to the system since the RF signal travels through the lossy human body. Another problem is that the size of the antenna is limited by the size of the header (at least for devices where the antenna is to be fitted inside the header.) Ideally, the antenna should have a length equal to a quarter wave length of the operating frequency (which is typically near 400 MHz), but it is difficult to design an antenna that fits within a device header while achieving that length. Hence, for antennas to be housed in the device header, the quarter wavelength constraint can result in an antenna much smaller than needed for optimum performance. Another issue is that the antenna should have an input impedance that is the complex conjugate of impedance of the internal circuitry of the device so maximum power transfer can take place. If the impedance of the antenna is too low or too high, additional mismatch losses will occur, which will decrease signal power.
FIG. 1 illustrates an antenna 2 that attempts to meet these requirements using a folded monopole design commonly known as an “Inverted L antenna” for use within the header 4 of an exemplary CRMD 6. The Inverted L is a monopole that ideally should be sized to a quarter wavelength of its operating frequency with a 90-degree bend to resemble a downward facing L. The antenna can fit within a fairly small header volume but suffers from very low input impedance. Also, this antenna is best suited for higher gigahertz (GHz) frequency applications where the necessary antenna length for resonance is relatively short. At 400 MHz, implementing an Inverted L antenna becomes impractical for implantable device purposes, as this would require a very long antenna that would not fit within the header. To solve the impedance issue, an extra branch 7 can be connected to the Inverted L and shunted to ground. This topology, shown in FIG. 2, is known as the “Inverted F antenna.” (An Inverted F antenna design is discussed, for example, in U.S. Pat. No. 7,047,076 to Li et al., entitled “Inverted-F Antenna Configuration for an Implantable Medical Device.”) The extra shunt connection provides a larger input impedance for matching purposes but the Inverted F still suffers from lack of adequate length for practical applications wherein the antenna must fit within the header of a relatively small CRMD.
The parent application cited above (entitled “Inverted E Antenna with Capacitance Loading for use with an Implantable Medical Device”) presented an improved antenna, particularly for MICS/MedRadio applications, that addressed these and other issues of predecessor designs. Briefly, the application described, inter alia, an antenna with an inverted E shape for mounting within the header of an implantable medical device. The antenna has three branches extending from a main horizontal arm: a capacitive branch connecting one end of the main arm to the case via a capacitive load; an RF signal feed branch connecting a middle portion of the main arm to the internal RF components of the device via a feedthrough; and an inductive branch connecting the opposing (far) end of the main arm to the case to provide a shunt to ground. The E-shaped configuration and the provision of capacitive loading allows for cancellation of inductance to bring the antenna into resonance and to provide optimal radiation efficiency as well as to provide for impedance with no reactive component. In one particular example, capacitive loading was achieved by installing a discrete capacitor along one of the branches of the antenna. In another example, the branch instead ended in a flat plate mounted via an epoxy dielectric to the case of the device so that the plate, the epoxy and the adjacent portion of the case collectively formed a parallel plate capacitor. During device design, capacitance could be set by selecting the size of the plate, the distance from the plate to the case and the electrical characteristics of the dielectric.
Although the inverted E antenna of the parent application has many advantages over predecessor designs, further room for improvement remains. For example, the use of a discrete capacitor adds to the cost of the device, particularly when a bio-compatible epoxy is required. The use of a flat plate at the end of one of the branches to provide capacitance (in conjunction with the adjacent portion of the case) requires high precision tooling to maintain the spacing between plate and the device “can” in order to produce the correct capacitance and achieve the desired RF performance. Proper spacing between the plate and the device housing can be difficult to achieve with a pre-cast header because there is considerable variance/tolerance in each individual header and also variance/tolerance in the attachment of the header to the can. Still further, the header epoxy used as the dielectric material can have inconsistencies. For example, the epoxy can develop bubbles during manufacture and can saturate with bodily fluid (saline) over time. Moreover, the header itself may shrink or warp, which can change the capacitance value created by the epoxy between the can and the plate. As a result the capacitance value may not be stable as desired over time. Still further, the plate needs to be fairly large to create an appropriate capacitance value (given the typical requirement for approximately 0.038″ of spacing between the can and the plate.) The reason for this spacing is that clearance is needed to attach (i.e. backfill) the header to the housing with epoxy. A smaller gap would reduce the size of the plate but then it could be difficult for epoxy to flow consistently between the header and the can, which presents a manufacturability issue. Also, epoxy can be a relatively poor dielectric for a capacitor because it has a low dielectric constant, such that the plate size has to be larger for a given capacitance.
Accordingly, it would be desirable to provide improvements to the inverted E antenna design and it is to this end that aspects of the present invention are generally directed.